I have just tried to book the 2 for 1 rate at the CP Kensington and here's what I got as total price for a room listed at 147.50 pounds and its non refundable or changeable. Does the rate rule highlighted mean the discount will be applied at the time of checkin?:
from the main booking page:
£147.50 GBP
Average Nightly Rate JUNIOR QUEEN SUITE NONSMOKING
Rate Type: Exclusive 2 for 1
Additional Rate Information: PURCHASE 1 NIGHT(S) AND GET 1 NIGHT(S) FREE
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From the detailed booking page:

Rate: £295.00 (GBP) per room, per night CONVERT CURRENCY
View Estimated Total Price †

Fri 19 Feb 2010 £295.00 GBP
per night (1 room(s))
Sat 20 Feb 2010 - Sun 21 Feb 2010 £0.00 GBP
per night (1 room(s))
Total Tax † £51.63 GBP
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Estimated Total Price †: £346.63 GBP


EFFECTIVE: 20 Feb
Rate Description: 2 night minimum stay required. Special Savings! Reservations are final and require full prepayment for the entire stay. Credit card is charged between time of booking and day of arrival. No refunds if cancelled or changed.
Deposit Required: A deposit for the entire stay is due at time of booking.

Tax: : 17.5% per night not included in rate effective 19 February, 2010 thru 21 February, 2010
VAT

Early Departure Fee: £50.00 (GBP)
Daily Parking Fee: £35.00 (GBP)
Pet Policy: Guide dogs are acceptable.
Room Type: JUNIOR QUEEN SUITE NONSMOKING - 2 PERSON(S) MAX PER ROOM
Number of Rooms: 1
Number of Persons: 1 ADULT(S) 0 CHILD(REN)
Rate Rules: MAXIMUM NUMBER OF DISCOUNTED NIGHTS IS 1. MUST STAY AT LEAST: 2 NIGHTS
MUST STAY NO MORE THAN: 2 NIGHTS

Check-in Time: 02:00 PM
Check-out Time: 12:00 PM
Modify or Cancel Policy: Canceling your reservation or failing to arrive will result in forfeiture of your deposit. Taxes may apply. Failing to call or show before check-out time after the first night of a reservation will result in cancellation of the remainder of your reservation.